Fabrication of One-dimensional Silicon Nano-wires Based on Proximity Effects of Electron-beam Lithography
Abstract One-dimensional silicon nanowire structures have been successfully made by using the proximity and accumulation effects of electron-beam (e-beam) lithography. Wire structures are fabricated in a thin poly silicon layer on a silicon substrate with a 400 nm buried SiO2. Measurements of the current-voltage characteristics at various temperatures from 4 K up to 300 K show significant nonlinearities and single-electron effect behavior. The blockade size is significantly affected by thermal effects, oscillations of the blockade, and the conductivity dependence on the gate potential.
AbstractThe SIA roadmap predicts mass production of sub-100 nm resolution circuits by 2006. This not only imposes major constraints on next generation lithographic tools but also requires that new resists capable of accommodating such a high resolution be synthesized and developed concurrently. Except for ion beam lithography, DUV, X-ray, and in particular electron beam lithography suffer significantly from proximity effects, leading to severe degradation of resolution in classical resists. We report a new class of resists based on organic/inorganic nanocomposites having a structure that reduces the proximity effects. Synthetic routes are described for a ZEP520®nano-SiO2 resist where 47nm wide lines have been written with a 40 nm diameter, 20 keV electron beam at no sensitivity cost. Other resist systems based on polyhedral oligosilsesquioxane copolymerized with MMA, TBMA, MMA and a proprietary PAG are also presented. These nanocomposite resists suitable for DUV and electron beam lithography show enhancement in both contrast and RIE resistance in oxygen. Tentative mechanisms responsible for proximity effect reduction are also discussed.
AbstractNovel arrays of gold nanoparticles with sulfur containing fullerene nanoparticles were self-assembled through the formation of Au-S covalent bonds. Disulfide functional groups were introduced into C60 molecule by reacting propyl 2-aminoethyl disulfide with C60. The two dimensional(2D) arrays were formed at the interface of aqueous phase of gold particles and organic phase of fullerene particles as a blue transparent film. TEM images showed that the fullerene spacing between adjacent Au(~10 nm) particles was about 2.1±0.4 nm, which was consistent with the result of 2.18 nm by molecular molding calculations(MM+). The arrays were deposited on the top of pairs of gold electrodes to form 2D colloidal single electron devices. The electrode pairs were made by electron beam lithography techniques, and the separation between tips of the two electrodes in a pair was less then 100 nm. Transport measurements at low temperatures exhibited Coulomb-Blockade type current-voltage characteristics, the lower the temperature the more pronounced the Coulomb gap. Also, step-by-step method was used to assemble one-dimensional(1D) array of gold nanoparticles with fullerene derivative between two electrodes spaced with 15 nm. The Coulomb blockade behavior of 1D arrays was clearer than that of 2D arrays.
This work applies combination of Direct Tunneling model and BSIM4 based ITAT model to explain the leakage of electrons from charged nanocrystals to p-type silicon substrate in data retention condition, for an ultra-thin tunnel oxide, low voltage programmable silicon nanocrystal based flash gate stack. Basic expressions of these models are modified to incorporate the nanocrystals related charge leakage in idle mode. The concept is supported by simulating these models and comparing them with the experimental data. Transition of electrons is considered as a result of Direct Tunneling and their trapping de-trapping via water related hydrogen traps. However, it is found that modified ITAT mechanism is the dominant one. Flat-band voltage shift profile fits accurately with the model with an extrapolated 10 years device lifetime without memory closure. 3 nm thick tunnel oxide and 100 nm sized nanocrystal fabrication with Electron Beam Lithography are main features of the devices.
Two prominent problems of electron beam lithography are slow throughput and proximity effects. The former arises from the serial nature of the exposure process; the current available in a beam of given resolution is limited by electron optical considerations and the resist sensitivity is limited by material considerations such that a dose of 1 μC/cm2 at 20 kV is required for the most sensitive resist and ten times that dose if high resolution is required.Proximity effects are caused by electrons scattered through lateral distances greater than the resolution of the pattern; a 20 keV electron in silicon has a range of about 3 μm whereas feature sizes are often less than 1 μm. Lowering the energy of the exposing electrons to, say, 2 keV would lower the electron range to less than 0.1 μm in silicon and thus effectively eliminate proximity effects as far as semiconductor circuit fabrication is concerned.
Nano-patterned crossbar structures were fabricated as test structures for the development of nanoelectronic devices based on functional molecules. The crossbar structures serve as a platform for testing electronic properties of molecules and their interface to metal electrodes. The fabrication of the crossbar structures involved electron-beam lithography of sub-100-nm features aligned to electrodes pre-patterned by UV lithography and the deposition of and pattern transfer into an intermediate layer. The molecules to be tested were self-assembled as a monolayer on the nano-patterned area. The top electrode structures were subsequently deposited on top of the intermediate layer. The crossbar architecture allows measuring the current-voltage characteristics across the molecules for each crossing point individually.